1. Field of the Invention
This invention generally relates to a method and apparatus for combating co-channel NTSC interference for digital TV transmission, and more particularly, to a receiver apparatus having a simplified rejection filter.
2. Discussion of the Related Art
The Federal Communications Commission (FCC), through its Advisory Committee on Advanced Television Service, and cable television testing organizations, such as CableLabs, have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing the Motion Picture Experts Group (MPEG) algorithms or variations thereof.
The FCC plans to test and approve an advanced television (ATV) standard for terrestrial broadcasting comprising, for example, high definition television (HDTV) and standard definition (SDTV) digital signals for terrestrial broadcasting. Although the specifics of the standard are yet to be fully tested and agreed upon, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach. The new HDTV signals will have to fit into currently unused television channels (so-called "taboo" channels) and initially co-exist with conventional analog television signals (NTSC) without co-channel interference.
NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonymously with "conventional" to represent conventional television in general.
In 1994, the FCC tested the so-called "Grand Alliance" system, a system which is being developed cooperatively by the corporate sponsors who were involved in a first round of individual digital proposals tested by the FCC in 1991 and 1992. This newly developed system proposes to take the best features from those systems already tested. This is being done in order to present a single optimum system for FCC approval as the U.S. standard.
The Grand Alliance has decided on a coding algorithm which will comply with the source coding standards proposed by MPEG. In addition, an RF transmission approach developed by Grand Alliance member Zenith Electronics Corporation was selected by the Grand Alliance. The RF transmission approach utilizes multi-level vestigial sideband (VSB) modulation which is described in "VSB Transmission System: Technical Details", Feb. 18, 1994.
The parent application, U.S. Ser. No. 08/230,360, filed Apr. 20, 1994, from which the instant application depends, describes an improved approach to combatting co-channel interference. The approach described in U.S. Ser. No. 08/230,360 utilizes an optimal NTSC "rejection filter" at the receiver in conjunction with an optimal precoder at the transmitter. The use of such a combination of a generalized "rejection" filter and precoder is in contrast to a use of a comb filter in a receiver as proposed by Zenith. The approach of the parent application further solves a number of problems and improves performance when both co-channel NTSC and additive white Gaussian noise (AWGN) are present. In addition, a method of using a generalized precoder and a corresponding decoder, for the case when uncoded QAM/VSB modulation is used, is described in "New Automatic Equalizer Employing Modulo Arithmetic", Electronic Letters, pp. 138-139, March 1971, by M. Tomlinson, and in "Matched-Transmission Technique for Channels With Intersymbol Interference", IEEE Transactions on Communications, vol. COM-20, no. 4, pp. 774-780, August 1972, by H. Harashima and H. Miyakawa.
While an improved overall performance of a transmitter/receiver system can be obtained when utilizing an optimal NTSC rejection filter and an associated precoder, it is however at the expense of some performance when only AWGN is present and when a co-channel NTSC spectrum is absent. This loss in performance is due to the fact that the NTSC rejection filter is implemented in the receiver at the decoder, which causes noise enhancement as a result of noise also passing through the NTSC rejection filter in the receiver.
Optimal rejection filters with the proposed constraints are disclosed in the parent application and also in U.S. patent application Ser. No. 08/271.810, filed on Jul. 7, 1994. The optimal rejection filters were designed to achieve a specific noise enhancement. From simulations, it has been determined that the filters with noise enhancement less than 0.4 dB reduce co-channel interference to a significant extent.
Noise enhancement alone, however, does not specify the overall loss in performance in AWGN. Simulations have shown that this loss in performance in AWGN could be between 0.3-0.8 dB depending on a choice of the trellis code and an amount of desired interference rejection, as disclosed for example, in the parent application of the present invention. As taught therein, when the transmitter utilizes a specific precoding, then for optimal performance, the receiver must also utilize the rejection filter for which the precoding was implemented at the transmitter. One problem with the later is that such an optimal rejection filter adds complexity in the receiver.